1. Field of the Invention
The present invention relates to flexible bearings, and more particularly, to an improved method for the fabrication thereof.
2. Description of the Prior Art
In the fabrication of flexible bearings in the prior art, it is known to utilize a lamination comprised of alternate layers of an elastomeric material and rigid reinforcement shims that are stacked and bonded together. With elastomer as both the top and bottom layers, the lamination is positioned between and bonded to metallic end rings. One end ring may comprise the thrust-nozzle ring of a rocket motor and the other the rocket case mounting ring. The lamination is characterized in that it is flexible in directions parallel to the layers, but is relatively unyielding in directions perpendicular thereto.
This type of flexible bearing has many uses in addition to mounting a movable thrust nozzle to a rocket motor case, including applications to motor mounts and bridge abutments. In its application to mounting a movable thrust nozzle to a rocket motor case, the flexible bearing is annular in form. Additionally, the layers of elastomer and the rigid reinforcement shims are shaped to conform to the surfaces of concentric spheres thereby to enable the thrust nozzle to be rotated about a fixed point. This is desirable for precision control of the rocket.
A typical bearing of this type having practical application for mounting a movable thrust nozzle to a rocket motor case is disclosed in U.S. Pat. No. 3,941,433 that was issued on March 2, 1976 to William T. Dolling et al. and is assigned to the assignee of the present invention.
High temperature steel is stated in U.S. Pat. No. 3,941,433 to be a preferred material for the rigid reinforcement shims. Reinforcement shims made of materials other than steel are also known. Thus, reinforcement shims have been fabricated from non-metallic materials such, for example, as glass cloth or other suitable fabric material. Such reinforcement shims contain, in addition to cloth, a resin or hardening material.
In the fabrication of reinforcement shims from non-metallic materials for mounting a movable thrust nozzle to a rocket motor case, it has been the practice in the prior art to separately prefabricate, in cured form, each of the plurality of rigid reinforcement shims that are employed in the flexible bearing lamination. An individually associated mold has been required for each of the reinforcement shims because each reinforcement shim conforms to the surface of an individually associated sphere having its own, unique radius. Thus, for example, in a lamination having nine reinforcement shims in the stack, nine separate and different molds are required for the prefabrication of the shims.
This prior art practice involving a multiplicity of molds for the laminate of each flexible bearing being fabricated significantly adds to the cost of the necessary tooling in addition to being labor intensive. Such tooling and labor costs escalate and become particularly onerous where fabrication of flexible bearings of several different sizes is involved. This is because of the numerous molds that are required to be used and the necessity for guarding against mixup of the many prefabricated reinforcement shims being produced. Additionally, the rejection rate of laminations made from prefabricated rigid reinforcement shims has been undesirably high. As a result, a very thorough inspection of each prefabricated reinforcement shim for the flexible bearing has been necessary to determine if it is in conformance with the required standards for the specific use for which it has been fabricated. This has significantly added further to the cost of production.
A method improving upon such prior art practice in the fabrication of flexible bearings and which enables a substantial reduction in the cost of tooling and labor and eliminates also the need for inspection of prefabricated reinforcement shims is disclosed in my U.S. Pat. No. 4,708,758 and is assigned to the assignee of the present invention.
The method there involved for making a flexible bearing, wherein the surface of each of first and second end rings adjacent the laminate and the surfaces of the layers of elastomer and rigid reinforcement shims of the lamination conform to surfaces of concentric spheres, comprises steps of:
(1) fitting together the end rings and laminate materials into an assembly of desired geometry with the resin impregnated fabric material being uncured, where prior to fitting together the end rings and laminate into an assembly, each of the layers of resin impregnated fabric material is formed into a reinforcement preform by pressing and compacting, wherein the reinforcement preforms are formed one at a time in a first mold configured to make the surfaces of the preforms conform to the surfaces of concentric spheres, and wherein the first mold is placed in a press for effecting pressing and compacting each preform; and PA0 (2) subjecting the assembly to heat and pressure thereby to cause cure and vulcanization of the elastomeric material and reinforcement shims. PA0 (a) preparing the reinforcement material in the form of a continuous helical arrangement; PA0 (b) placing the first end ring in the bottom of a mold with the laminate engaging surface thereof facing upwards; PA0 (c) coiling the continuous helical arrangement of reinforcement material on the laminate engaging surface of the first end ring, coaxially therewith, with layers of elastomeric material alternating with layers of reinforcement material, to form the laminate; PA0 (d) placing the second end ring in the mold with the laminate engaging surface thereof in engagement with the laminate, coaxially therewith, and with the first end ring; and PA0 (e) debulking and curing the assembly.
In one embodiment disclosed in U.S. Pat. No. 4,708,758, six "quarter circle" plies of resin impregnated glass cloth, constituting a total of twenty-four patterns, are used for each reinforcement shim layer. The elastomer for the lamination comprises calendered natural or synthetic rubber pads having good elastic and holding properties over a wide temperature range.
While the method disclosed in my aforementioned patent has effected a substantial improvement in the cost of fabricating flexible bearings, there is, however, a need and a demand for effecting further improvement. Specifically, there is a need for effecting improvement in the method of fabricating flexible bearings in order to eliminate the need of mold tooling to form individual reinforcement shims, and in addition, the labor involved in producing individual reinforcement shim preforms thereby further to simplify and reduce production costs. The present invention was devised to fill the technological gap that has existed in the art in these respects.